This invention generally relates to a system and method for mapping the magnitude of residual surface stresses of a machine component, and is particularly concerned with the mapping of residual compressive surface stresses of a ferromagnetic valve ring with an eddy current probe in order to assure the fatigue life of the ring.
Suction ring valves are critical components of compressors used in refrigeration units. As such, they have to be extremely reliable. The service life of compressor valve depends upon its resistance to fatigue. To maximize the fatigue life of such rings, the manufacturers (after die cutting and heat treating the rings) tumble them in order to induce surface compressive stresses.
Ideally, a finished valve ring should be free of superficial defects, having a good surface finish, have good flatness and round edges, and most importantly should have a uniform high compressive residual stress of at least 70 ksi at all points on its surface in order to maximize the fatigue strength and hence durability of the valve ring. However, flatness requirements sometime requires the valve manufacturer to straighten the ring after tumbling. In such a straightening operation, the ring is essentially bent along one of its diameters in order to counteract an unwanted mount of camber or curl in the ring. Unfortunately, such a bending of the ring can reduce or neutralize completely the compressive stresses present in the surface of the ring along the diameter where it is bent, thereby substantially shortening its fatigue life. Worse yet, such bending can introduce tensile stresses along the bending diameter which can shorten the fatigue life of the ring even more. Consequently, such straightened valve rings will tend to prematurely crack along the diameter where they were bent in order to counteract the unwanted camber or curl that they were initially manufactured with. When such cracking occurs, the operation of the refrigeration compressor that such rings are used in fails.
While x-ray diffraction techniques are known which are capable of mapping the pattern of compressive stresses that exist around the circumference of such ring-shaped components, such techniques are slow and expensive to implement. Thus they are not well suited for the rapid inspection of a batch of 100 or 1000 compressor rings.
Clearly, there is a need for a system for mapping the residual surface stresses present in ferromagnetic components such as valve rings that is quicker, easier, and more reliable than prior art x-ray diffraction techniques. Ideally, such a system should be capable of quickly and reliably inspecting hundreds or thousands of such valve rings so that rings with inadequate compressive surface stresses may be discarded before being assembled into refrigeration compressors.